Fsck for lazy objects, and (now) actual invocation of loader

Besides review changes, this patch set now includes my rewritten lazy-loading sha1_file patch, so you can now do this (excerpted from one of the tests):

test_create_repo server
test_commit -C server 1 1.t abcdefgh
HASH=$(git hash-object server/1.t)

test_create_repo client
test_must_fail git -C client cat-file -p "$HASH"
git -C client config core.repositoryformatversion 1
git -C client config extensions.lazyobject \
"\"$TEST_DIRECTORY/t0410/lazy-object\" \"$(pwd)/server/.git\""
git -C client cat-file -p "$HASH"

with fsck still working. Also, there is no need for a list of promised blobs, and the long-running process protocol is being used.

Changes from v1:
- added last patch that supports lazy loading
- clarified documentation in "introduce lazyobject extension" patch (following Junio's comments [1])

As listed in the changes above, I have rewritten my lazy-loading sha1_file patch to no longer use the list of promises. Also, I have added documentation about the protocol used to (hopefully) the appropriate places.

This is a minimal implementation, hopefully enough of a foundation to be built upon. In particular, I haven't added the environment variable to suppress lazy loading, and the lazy loading protocol only supports one object at a time.

Other work
----------

This differs slightly from Ben Peart's patch [2] in that the lazy-loading functionality is provided through a configured shell command instead of a hook shell script. I envision commands like "git clone", in the future, needing to pre-configure lazy loading, and I think that it will be less surprising to the user if "git clone" wrote a default configuration instead of a default hook.

This also differs from Christian Couder's patch set [3] that implement a larger-scale object database, in that (i) my patch set does not support putting objects into external databases, and (ii) my patch set requires the lazy loader to make the objects available in the local repo, instead of allowing the objects to only be stored in the external database.

[1] https://public-inbox.org/git/[email protected]/
[2] https://public-inbox.org/git/[email protected]/
[3] https://public-inbox.org/git/[email protected]/

Jonathan Tan (5):
environment, fsck: introduce lazyobject extension
fsck: support refs pointing to lazy objects
fsck: support referenced lazy objects
fsck: support lazy objects as CLI argument
sha1_file: support loading lazy objects

 Documentation/Makefile                             |   1 +
 Documentation/gitattributes.txt                    |  54 ++--------
 Documentation/gitrepository-layout.txt             |   3 +
 .../technical/long-running-process-protocol.txt    |  50 +++++++++
 Documentation/technical/repository-version.txt     |  23 +++++
 Makefile                                           |   1 +
 builtin/cat-file.c                                 |   2 +
 builtin/fsck.c                                     |  25 ++++-
 cache.h                                            |   4 +
 environment.c                                      |   1 +
 lazy-object.c                                      |  80 +++++++++++++++
 lazy-object.h                                      |  12 +++
 object.c                                           |   7 ++
 object.h                                           |  13 +++
 setup.c                                            |   7 +-
 sha1_file.c                                        |  44 +++++---
 t/t0410-lazy-object.sh                             | 113 +++++++++++++++++++++
 t/t0410/lazy-object                                | 102 +++++++++++++++++++
 18 files changed, 478 insertions(+), 64 deletions(-)
 create mode 100644 Documentation/technical/long-running-process-protocol.txt
 create mode 100644 lazy-object.c
 create mode 100644 lazy-object.h
 create mode 100755 t/t0410-lazy-object.sh
 create mode 100755 t/t0410/lazy-object

2.14.0.rc1.383.gd1ce394fe2-goog